Today high capacity communication by optical fiber is a common phenomenon. Indeed, optical networks using optical fibers have become more widespread in recent years as they are suitable for communication of various multimedia services and similar being accessed via the broadband signal transmission over the Internet or similar.
The interest has subsequently increased for long and ultra long distance transmission with high spectral efficiency in the optical domain.
A problem related to most electro optical modulators that are used in this connection is that a two sided spectrum around the optical carrier frequency is created when a set of original RF channels are converted into the optical domain. Here, the spectrum of the original RF channels is mirrored around the optical carrier such that the spectrum of the original RF channels is positioned at one side of the optical carrier whereas a copy of the spectrum of the original RF channels is positioned at the other side of the optical carrier.
This creates at least two problems: 1) since two versions of the RF channel are generated this doubles the optical bandwidth compared to the electrical bandwidth required for the original RF channel which reduces the optical spectral efficiency; 2) even in systems that do not require high optical spectral efficiency, transmission of multiple versions of the same signal (e.g. two versions of the RF channel as indicated above) may cause severe signal degradation due to Chromatic Dispersion (CD) if both versions are detected in the receiver. This may e.g. be the case if direct detection techniques are used, e.g. if a square law detector is used. Also, nonlinear effects may cause coupling between closely spaced subcarriers and thus redundant signals, e.g. such as another version of the original RF channel as indicated above, should be avoided in the system.
Some of the problems indicated above may be dealt with by using Optical Single Side-Band (OSSB) techniques, e.g. by removing one half of the optical spectrum by an optical filter. However, OSSB technologies reduce the usable optical bandwidth from the modulator with a factor two (2), since one half of the optical spectrum is removed and left unused. Thus by using optical modulators available today the optical bandwidth that can be covered by a modulator may e.g. be about 50 GHz, even though the modulator as such may cover 100 GHz.